In wideband land-mobile, radio communication systems utilizing Time Division multiplexed Multiple Access (TDMA) control, such as the cellular system proposed for use in Europe, an RF channel is shared (time-division-multiplexed) among numerous subscribers attempting to access the radio system in certain ones of various time-division-multiplexed time slots.
FIG. 1 is a specification of the TDMA RF protocol of the GSM Pan-European Digital Cellular System, including the access burst specification.
As illustrated in FIG. 1, eight time slots are arranged into periodically repeating frames. Subscriber stations are given information about the transit time of their transmissions to permit them to advance or retard their transmissions to synchronize with the base station such that nearly the entire time slot is filled with digital information of 148 bits. However, subscribers attempting to initially access the system have no information about the propagation delay of their transmissions. Thus, time slot 0 in certain frames is reserved for those attempting to initially access the system. Their burst transmissions used to initially access the system are foreshortened to allow for transmissions ranging from a huge signal transmitted from right next to the base station antenna to a moderate signal transmitted from moderate distance to a weak signal transmitted from the boundary of the cell and to allow for their concomitant propagation delays. The base station receiver must be prepared to receive a signal virtually anytime during the 6/10ths of a milliseconds time slot, must get the gain of a signal that is unpredictable by as much as 100 dB (depending upon distance) within the dynamic range of the receiver (say 48 dB) during the 30 microseconds (8 bits at 270 kilobits/second) of transmitter key up and in time to detect the signal envelope during the 30 microseconds (8 bits) of head information and correlate 152 microseconds (41 bits) of synchronizing information before the 133 microseconds (36 bits) of useful information disappears. This situation provides a formidable challenge indeed.
Ordinarily, the base station would miss the access burst of the subscriber because the gain would either be too low and the detection and correlation would false on noise or the gain would be too high and the signal would saturate the receiver. The subscriber, failing to receive acknowledgement and propagation delay information for subsequent accesses would gradually reduce its power on subsequent retries and then increase its power again until finally the base station and subscriber happened upon the right combination.
This invention takes as its object to overcome these challenges and to realize certain advantages presented below.